Interpretive Summary: The heat and drought tolerant annual legume pigeon pea planted between winter wheat crops has been promoted as a potential source of grain and animal feed for southern Great Plains farmers and cattlemen seeking to diversify their agricultural options in the summer. Unfortunately, pigeon pea seedlings compete poorly against the rapid growth of warm-season annual weeds. Currently, no herbicides are labeled for use on pigeon pea grown in the USA and the risk of crop failure is high without weed control. We investigated the effects of applications rates of herbicides (pre- and post-emergence) on weed suppression, pigeon pea dry matter, and carry-over effects on a winter wheat crop. Pigeon pea productivity ranged from 750 to 2,290 lb dry matter/acre with sethoxydim and the untreated control <= metribuzin <= sulfentrazone <= hand weeded control <= imazapic. Although imazapic gave the best weed control among the herbicides, it reduced wheat productivity by more than 60% and should not be used unless imidazolinone herbicide tolerant wheat cultivars are planted. All of the herbicides we used carry grazing restrictions for annual legumes on their USA labels (ranging from 40 d to do not feed) and could prevent adoption by southern Great Plains producers, if they wish to use pigeon pea for hay or possibly as forage once labeled these herbicides are for use with pigeon pea.

Technical Abstract:
Weed control is required before the heat and drought-tolerant pigeon pea (Cajanus cajan (L.) Millsp.) can be reliably grown in the USA southern Great Plains as a potential source of grain and animal feed between annual plantings of winter wheat (Triticum aestivum L.). Currently, no herbicides are labeled for use on pigeon pea grown in the USA. Effects of applications (1X and 2X rates) of herbicides (pre-emergence, sulfentrazone + chlorimuron and metribuzin; post-emergence, imazapic and sethoxydim) on weed suppression, pigeon pea dry matter, and carry-over effects on a winter wheat crop were determined. The most abundant summer weeds were broadleafs, and all herbicide treatments, except sethoxydim (grass herbicide), reduced weed densities compared to untreated plots without adversely affecting pigeon pea stands. Imazapic treatments provided the most effective weed control. Overall average pigeon pea dry matter ranged from 750 to 2560 kg/ha with sethoxydim and the untreated control <= metribuzin <= sulfentrazone <= hand weeded control <= imazapic. Compared to the hand-weeded control, imazapic treatments greatly reduced wheat dry matter (1X, 65% and 2X, 91%) and grain yield (1X, 59% and 2X, 93%). Imazapic should not be used unless nontransgenic imidazolinone herbicide tolerant wheat cultivars are planted. While the other herbicides decreased negative effects of weeds on pigeon pea dry matter without greatly affecting productivity of a following wheat crop, appropriate labels for each of these herbicides will be required prior to their use by southern Great Plains pigeon pea producers.